This invention relates to Automatic Gain Control (AGC) circuitry and more specifically to AGC circuitry having particular utility in pulse radar systems.
The design of automatic gain control circuitry for pulse radar systems is greatly complicated by conflicting requirements which the AGC must meet in this particular application. The problem become especially acute where the radar receiver gain is to be controlled by sampling the received signal through only a very brief portion of the pulse repetition period or of some submultiple of that period, and complete loop correction is to be accomplished in the inter-pulse period. In a typical radar system employing this sampling technique, for example, the background noise in the receiver IF channel may be sampled for about 10 microseconds of the "no target" portion of the inter-pulse period, thereby assuring that receiver gain is controlled substantially wholly by background noise and that the target signal returns themselves do not affect amplifier gain as of course would not be desired.
Because the pulse repetition period is itself short, commonly of the order of 50 milliseconds, the sampling period must be kept very short and as previously indicated may be of the order of 10 microseconds. The AGC loop must have relatively high gain, so as to be able to complete any correction indicated to be necessary by the sampled signal within perhaps 30 microseconds after the sample is taken. This in turn requires that the AGC loop have a relatively very short time constant and, consequently relatively very high bandwidth. For loop stability, however, the loop bandwidth should be relatively very narrow, substantially less than the sampling frequency which in this case is the radar pulse repetition frequency or a submultiple thereof. Thus there arises a severe conflict between those requirements of the AGC loop necessary to achieve the desired system gain control characteristic and those essential to loop stability.
The present invention is directed to the provision of automatic gain control circuitry which is capable of satisfying the conflicting requirement of a loop response time that is very short as compared with the time between samples. It is also an objective of the invention to provide automatic gain control circuitry for pulse radar system applications in which the AGC loop is separate from and not closed through the controlled-gain amplifier itself, thus enabling loop operation with sampling rate periods much greater than the loop time constants yet without loop instability. The provision of such separate AGC loop also is of advantage in enabling control of gain of more than a single amplifier channel, as may be advantageous with multichannel IF amplifiers.